convert webp lossless test to a non alpha image

thanks to @kristerkari for [pointing it out](https://github.com/Modernizr/Modernizr/issues/1023#issuecomment-22939414).

conveniently shaves a few bytes off, as well.

```bash
  convert -size 1x1 canvas:white white.jpg
  cwebp -lossless -noalpha /private/tmp/white.jpg -o losslessNoAlpha.webp
```